Flexible printed circuit boards (FPCs) and rigid-flexible printed circuit boards constitute an increasingly strong growth area in printed circuit board manufacturing, as they offer numerous advantages over rigid circuit panels. FPC's and rigid-flexible printed circuit boards offer electronic equipment manufacturers the advantage of flexibility and compact high density wiring with high reliability, weight reduction and an overall cost saving. FPC's have been in use since the late 50's/early 60's mainly in military/space applications, however more recently they are more commonly found in retail products such as cameras, mobile phones and MP3 players.
The current manufacturing techniques for producing FPC's are described in Printed Circuit Board Materials Handbook by Martin W. Jawitz, McGraw-Hill Professional, 1997, page 784. This highly detailed book explains FPC construction and describes/discusses the assembly steps involved.
Rigid-flexible circuit boards are constructed from a flexible inner layer onto which a rigid out layer is applied to one, or more typically both, sides. In order to combine the flexible part of a rigid/flex circuit board with the rigid part, the use of No Flow Prepreg is typically necessary to give adhesion between the surface layer (usually the coverlay) of the flexible part and the rigid part of the rigid/flex circuit board. “No Flow Prepregs” are solid materials which are used as adhesive layers in lamination processes. Typically they comprise a fabric reinforcement that has been pre-impregnated with a resin and partially cured. No Flow Prepregs have a very high melt viscosity so that during the lamination cycle in which heat and pressure is applied to the Prepreg to fully cure the Prepreg and form an adhesive bond, minimal flow of the liquefied resin is observed. Ideally, the flow of the resin is sufficient to enable a good bond to be formed but not enough to result in substantial “bleed” or “leakage” out from the area it is required. “No Flow Prepregs” may alternatively be referred to a “Low Flow Prepregs”. Two of these Prepregs are needed for each side and they need to be exactly cut to size with low tolerances. The adhesion between the rigid and flexible layers is typically achieved through another press cycle. After thermal cycling, the adhesion achieved using Prepreg is a potential weak point.
Adhesive layers are typically used in the preparation of laminate materials in electronic components, for example to bond flexible or rigid layers in the preparation of a multilayer laminate. Adhesives are also used in the preparation of electronic components to bond stiffener materials and/or heat sinks to the base material, such as to a circuit board base material.
Adhesives may be either supplied as liquid or as dry films in the form of sheets or rolls, either pre-applied to a laminate layer, such as on the back of a coverlay layer or as a separate sheet or roll of adhesive, the adhesive side or sides of such film often being coated with a release paper. The use of dry film adhesives involves high cost in terms of manpower, material consumption (including wastage) and energy time expenditure. These dry film sheets or rolls can only be applied using a time and cost intensive processes, especially if they are to be used only on selected areas of a panel. To apply adhesives in the form of dry sheets or rolls to the selected areas would require multiple process steps. For example:                1. Cutting those areas from a sheet or a roll by use of laser cutter or cut plotter.        2. Manually removing the protective layer from the adhesive side.        3. Positioning the adhesive (and the laminate layer if separate) onto the panel in manual operation with best possible accuracy (>0.8 mm).        4. Fixing the adhesive in manual process using a soldering tool.While it is possible to carry out steps 1-4 by use of automation (e.g. robots) saving significant manual labor and associated expenses, it can be appreciated that the cost of investing in and operating the multiple robots necessary to carry out these operations sequentially is extremely high.        
In the electronics industry, acrylic adhesives and epoxy adhesives (in both liquid and dry film form) are traditionally used. However, those traditional adhesives can encounter technical issues when used in combination with polyimide coverlays including humidity absorption, smearing, dust and limited accuracy of positioning which does not conform to modern technical requirements.
Other associated problems with using dry film adhesives for mechanical processes such as flexible printed circuits (FPCs) as well as other printed circuit boards (PCBs) and other electronic components include:                Limited accuracy (0.8 mm minimum) due to manual positioning of the films.        Dimensional problems across the z axis.        Foil instability due to manual mounting and press processes.        Compatible only with selective metallization processes. Acrylic adhesive bleeds therefore plasma desmear* required as an additional process step.        Technical problems arise with machining, drilling and routing which include smear and dust.        Each process cycle is very time consuming.*A general description of “smear” and “desmear” can be found in the following reference: A Comprehensive Guide to the Design and Manufacture of Printed Circuit Board Assemblies—Volume 2, William Macleod Ross, Electrochemical Publications, page 232.        
Polyamideimides are thermoplastic amorphous polymers that have exceptional mechanical, thermal and chemical resistant properties. These materials can be used in a variety of electronic applications including Printed Circuit Boards (PCBs), Photovoltaic's, Displays and Membrane Switches. As traditional polyimides used in these areas are applied as dry-film adhesive sheets, there is a limit to how these devices can be fabricated.
U.S. Pat. No. 7,364,799 (Toyo Boseki) and WO 2008/072495 (Toyo Boseki) disclose polyamideimide (PAI) resins for application to a flexible medal-clad laminate.
WO 2008/041426 (Hitachi) (also published as EP 2 070 961) discloses a polyamideimide (PAI) resin for a flexible printed circuit board. The PAI has at least one terminal functional group selected from a carbonyl group, an amino group, an acid anhydride group and a mercapto group to enhance the heat resistance.
Liquid compositions comprising solutions of polyamideimides are disclosed in W02008/041426A1 (republished in English as US2010/0170701 A1) and US 2007/0166559 A1 for use as adhesives and a coverlays for flexible printed circuit boards.
A problem exists in the use of polyamideimide (PAI) resins in that they show poor viscosity stability, this is due to the fact that residual isocyanate groups remaining from the synthesis can react with pendent carboxylic acid groups causing the viscosity to rise with time. If the ratio of isocyanate is reduced to counter this in the synthesis of the PAI then the amount of amide groups are reduced with an increase in the level of imide functionality which results in the solubility of the polymer being much reduced.